Magnetic sidewalls for write lines in field-induced mram and methods of manufacturing them

ABSTRACT

In one embodiment, there is provided a non-volatile magnetic memory cell. The non-volatile magnetic memory cell comprises a switchable magnetic element; and a word line and a bit line to energize the switchable magnetic element; wherein at least one of the word line and the bit line comprises a magnetic sidewall that is discontinuous.

The present application is a continuation of, and claims priority under35 U.S.C. §120 to, copending U.S. patent application Ser. No.14/101,512, filed on Dec. 10, 2013, which is a continuation of, andclaims priority under 35 U.S.C. §120 to, U.S. patent application Ser.No. 13/340,452, filed on Dec. 29, 2011, now U.S. Pat. No. 8,625,340,which claims priority under 35 U.S.C. §119(e) to provisional applicationSer. No. 61/428,161, filed on Dec. 29, 2010. The entire contents of eachprior-filed application are hereby expressly incorporated herein byreference.

FIELD

Embodiments of the invention relate to magnetic random access memory(MRAM) devices and methods for their manufacture.

BACKGROUND

Field-induced magnetic random access memory (MRAM) use a current-inducedmagnetic field generated around metal lines to write data in memorycells. In an MRAM cell one bit of data is stored in a magnetic tunneljunction (MTJ). In field-induced MRAM the MTJ sits in-between two metallines, the bit line and the word line. Normally, these lines areperpendicular to each other. To write binary data (“0” or “1”) in an MTJcell, enough current must go simultaneously through the bit line and theword line of that particular cell for a certain amount of time. Thesense in which the current flows in both metal lines sets a data valueof either a “0” or a “1” in the cell.

It is advantageous to MRAM technology to be able to write data in thememory cells with as low a current as possible. Lower current meanslower energy and voltage requirements for the memory device, smallertransistors (which may impact positively the memory density), and higherreliability of the metal lines employed in writing the cells is.

SUMMARY

According to one aspect of the invention, there is provided anon-volatile magnetic memory cell, comprising a switchable magneticelement; and a word line and a bit line to energize the switchablemagnetic element; wherein at least one of the word line and the bit linecomprises a magnetic sidewall that is discontinuous.

According to second aspect of the invention, there is provided memorydevice, comprising: an array of magnetic memory cells, each cellcomprising a switchable magnetic element; and a word line and a bit lineto energize the switchable magnetic element; wherein at least one of theword line and the bit line comprises a magnetic sidewall that isdiscontinuous.

Other aspects of the invention will be apparent from the writtendescription that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic drawing of metal lines with a) continuous andb) discontinuous magnetic sidewalls, in accordance with one embodimentof the invention.

FIG. 2 shows a 3×3 cell array in one MRAM cell configuration withcontinuous magnetic sidewalls in both bit-lines and word-lines, inaccordance with one embodiment of the invention.

FIG. 3 shows a 3×3 cell array in one MRAM cell configuration withdiscontinuous magnetic sidewalls in both bit-lines and word-lines, inaccordance with one embodiment of the invention.

FIG. 4 shows a cross-section through a metal line with magneticsidewalls composed of a magnetic layer sandwiched by two non-magneticlayers, in accordance with one embodiment of the invention.

FIG. 5 shows steps in a process flow for manufacturing metal lines withcontinuous magnetic sidewalls, in accordance with one embodiment of theinvention.

FIG. 6 shows steps in a process flow for manufacturing metal lines withdiscontinuous magnetic sidewalls, in accordance with one embodiment ofthe invention.

FIG. 7 shows steps in a process flow for manufacturing metal lines withcontinuous magnetic sidewalls, in accordance with another embodiment ofthe invention.

FIG. 8 shows steps in a process flow for manufacturing metal lines withdiscontinuous magnetic sidewalls, in accordance with another embodimentof the invention.

DETAILED DESCRIPTION

In the following description, for purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the invention. It will be apparent, however, to oneskilled in the art that the invention can be practiced without thesespecific details.

Reference in this specification to “one embodiment” or “an embodiment”means that a particular feature, structure, or characteristic describedin connection with the embodiment is included in at least one embodimentof the invention. The appearance of the phrase “in one embodiment” invarious places in the specification are not necessarily all referring tothe same embodiment, nor are separate or alternative embodimentsmutually exclusive of other embodiments. Moreover, various features aredescribed which may be exhibited by some embodiments and not by others.Similarly, various requirements are described which may be requirementsfor some embodiments but not other embodiments.

Although the following description contains many specifics for thepurposes of illustration, one skilled in the art will appreciate thatmany variations and/or alterations to said details are within the scopeof the present invention. Similarly, although many of the features ofthe present invention are described in terms of each other, or inconjunction with each other, one skilled in the art will appreciate thatmany of these features can be provided independently of other features.Accordingly, this description of the invention is set forth without anyloss of generality to, and without imposing limitations upon, theinvention.

Broadly, embodiments of the present invention disclose MRAM structureswith metal lines having magnetic sidewalls in different configurations.In a first configuration, the magnetic sidewalls are continuous andextend along the full length of a metal line. In a second configuration,the magnetic sidewalls are discontinuous and are located at portions ofmetal lines that are close to the MTJ cells. Advantageously, themagnetic sidewalls reduce the current in the word and bit lines neededto switch the MTJ cells. Embodiments of the present invention alsodisclose techniques for manufacturing the metal lines.

Referring now to FIG. 1(a), in a first configuration metal line 2 isshown having continuous magnetic sidewalls 4 extending along its entirelength. FIG. 1(b) shows the metal line 2 clad with discontinuous metalline, portions of which are indicated with reference numeral 6.

Referring now to FIG. 2, reference numeral 8 generally indicates a 3×3MRAM array, in accordance with one embodiment of the invention. In thearray 8, each MRAM cell includes a word line 10 and a bit line 12 with aMTJ stack/element 14 disposed at the intersections of the word and bitlines 10, 12. As will be seen, the word lines 10 have continuoussidewalls 10.1, whereas the bits lines 12 have continuous sidewalls12.1. Each MTJ 14 is connected to access circuitry (not shown) through abit line 10, a bottom electrode 16, and a stack 18.

Referring now to FIG. 3, reference numeral 19 generally indicates a 3×3MRAM array, in accordance with another embodiment of the invention. InFIG. 2, the same or similar reference numerals used in FIG. 1 are usedto indicate the same or similar components. As will be seen, the case ofthe embodiment of FIG. 3, the bit line 10 includes discontinuousmagnetic sidewalls, portions of which are indicated with referencenumeral 10.2. Likewise, the word line 12 includes discontinuous magneticsidewalls, portions of which are indicated with reference numeral 12.2.

In one embodiment, in the case of continuous magnetic sidewalls saidsidewalls are magnetically very soft. For this purpose, the magneticwalls may be made of NiFe, NiFeMo alloys or ultrasoft magneticmaterials. In one embodiment, the thicknesses of the magnetic layer inthe sidewalls are selected to keep the soft properties of the magneticsidewalls. For example, thin magnetic layers (much less than 10 nm) inthe sidewalls are avoided.

In the case of the configuration with discontinuous magnetic sidewalls,said sidewalls may be thin (<10 nm) so as not to overpower the MTJ.

In one embodiment, the aspect ratio of these patterned sidewalls is setcarefully and consistently across the memory device. Setting the aspectratio of the patterned walls involves considering the magnetic switchingfield of the cell, the cell stability against thermal fluctuations,stray magnetic fields, and half-select. In one embodiment, the patternedmagnetic sidewalls have an aspect ratio 1 or close to 1 with the longerside oriented along a top-bottom direction in FIG. 3. The magnetic fieldfor switching the cells as well as the cell stability tends to increasewith the aspect ratio of the magnetic sidewalls. For the magneticsidewalls it is preferable to use materials with very lowmagneto-crystalline anisotropy, like NiFe, NiFeMo or CoFeB alloys.

In one embodiment, the magnetic sidewalls may be made of several layers.FIG. 4 shows a cross-section through a metal line (word or bit) havingthree layer magnetic sidewalls. The layers include an outer layer 20, aninner layer 22, and a middle layer 21 sandwiched between the outer layer20 and the inner layer 22. The layers 20 and 22 are non-magnetic,whereas the middle layer 21 is the magnetic one. The purpose of theouter 22 and the inner 20 layer is to protect the integrity of themagnetic layer 21, so that its thickness is not affected by processing.The innermost layer 20 also helps protect the metal line 23 duringprocessing and helps reduce electro-migration in the metal line 23. Theouter and the inner layers may be composed of Ta, or other materialsthat fit the purpose.

Manufacturing of the magnetic sidewalls can be accomplished by differentmethods. In case of metal lines defined by etching a metallic layer;like AlCu and W lines, the process flow for manufacturing is shown inFIG. 5. The first step is to define the metal line 30 (FIG. 5 a), whichis shown in cross-section. In one embodiment the metal line 30 isdefined with the assistance of a hard mask, the remaining of which isdenoted as 31. The next step is the deposition of the layers 32composing the magnetic sidewalls, as shown in FIG. 5 b). In oneembodiment the deposition can be made by Physical Vapor Deposition(PVD). The next step (c) is the definition of the walls throughanisotropic etching of the deposited layers. In one embodiment this canbe accomplished with Reactive Ion Etching (RIE) using for example:chlorine gas mixed with argon. At this point the continuous magneticsidewalls are already defined along the metal lines. For discontinuousmagnetic sidewalls, after step c) a photo-lithography process follows,as shown in plan view in FIG. 6 d). This step is to protect withphoto-resist 40 the parts of the magnetic sidewalls that are going toremain. The next step (e) is the etching away of the exposed magneticsidewalls. After stripping the photo-resist (f), the discontinuousmagnetic sidewalls 41 are defined.

In the case of metallic lines defined by the Damascene method, like Culines, the process flow for manufacturing the magnetic sidewalls isshown in FIG. 7. Starting from the groove 50 etched in the dielectriclayer 51, the next step is the deposition of the layers 52 composing themagnetic sidewalls. In one embodiment the deposition can be made by PVD.The following step (c) is the definition of the walls throughanisotropic etching of the deposited layers. In one embodiment this canbe accomplished with RIE using chlorine gas mixed with argon gas. Thefollowing step (d) is the filling of the groove with metal and thechemical-mechanical polishing (CMP) down to the dielectric layer. Formetal filling a thin metallic seed layer 53 is deposited first. In adifferent embodiment the innermost metallic layer 54 deposited in b)serves as seed layer for metal filling. For that purpose, step c) isreplaced with step e), where the innermost deposited layer is left afterRIE. After that, step f) follows which implies metal filling and CMP.Either after step d) or after step f) the result is the definition ofcontinuous magnetic sidewalls along the metal lines. For discontinuousmagnetic sidewalls, after step c) or e) a photo-lithography processfollows, as shown in plan view in FIG. 8 g). This step is to protectwith photo-resist 60 the parts of the magnetic sidewalls that are goingto remain. The following step (h) is etching away the exposed magneticsidewalls. After stripping the photo-resist (i), the discontinuousmagnetic sidewalls 61 are defined.

One skilled in the art would be aware of the requirements andspecificities of the techniques mentioned above for the purpose ofmanufacturing the magnetic sidewalls. The manufacturing techniquesmentioned herein are not intended to limit the scope of the invention.

1-20. (canceled)
 21. A method for manufacturing a discontinuous magneticsidewall of at least one of a word line and a bit line, the methodcomprising: defining the at least one of the word line and the bit line;depositing layers on the at least one of the word line and the bit line;defining a magnetic sidewall; protecting discontinuous portions of thedefined magnetic sidewall, wherein non-protected portions define exposedportions; and removing the exposed portions.
 22. The method of claim 21,wherein said depositing layers is done by Physical Vapor Deposition(PVD).
 23. The method of claim 21, wherein the at least one of the wordline and the bit line is defined with assistance of a hard mask.
 24. Themethod of claim 21, wherein the magnetic sidewall is defined byperforming anisotropic etching of the deposited layers.
 25. The methodof claim 21, wherein the magnetic sidewall is defined by performingreactive ion etching of the deposited layers.
 26. The method of claim25, wherein said performing reactive ion etching of the deposited layersuses chlorine gas mixed with argon gas.
 27. The method of claim 21,wherein the discontinuous portions are rectangular.
 28. The method ofclaim 21, wherein the discontinuous portions have an aspect ratio of 1.29. The method of claim 21, wherein the discontinuous portions have athickness that is less than 10 nm.
 30. The method of claim 21, whereinthe layers comprise an inner layer, an outer layer, and a middle layersandwiched between the inner and outer layers.
 31. The method of claim30, wherein the middle layer is magnetic.
 32. The method of claim 30,wherein the inner and outer layers are non-magnetic.
 33. A method formanufacturing a discontinuous magnetic sidewall of at least one of aword line and a bit line, the method comprising: etching a groove in adielectric layer; depositing layers in the groove; defining a magneticsidewall; protecting discontinuous portions of the defined magneticsidewall, wherein non-protected portions define exposed portions; andremoving the exposed portions.
 34. The method of claim 33, wherein saiddepositing layers is done by Physical Vapor Deposition (PVD).
 35. Themethod of claim 33, wherein the magnetic sidewall is defined byperforming anisotropic etching of the deposited layers.
 36. The methodof claim 33, wherein the magnetic sidewall is defined by performingreactive ion etching of the deposited layers.
 37. The method of claim36, wherein said performing reactive ion etching of the deposited layersuses chlorine gas mixed with argon gas.
 38. The method of claim 36,wherein: the layers comprise an inner layer, an outer layer, and amiddle layer sandwiched between the inner and outer layers; and theinner layer is left at a bottom of the groove after the magneticsidewall is defined.
 39. The method of claim 33, wherein thediscontinuous portions are rectangular.
 40. The method of claim 33,wherein the discontinuous portions have an aspect ratio of
 1. 41. Themethod of claim 33, wherein the discontinuous portions have a thicknessthat is less than 10 nm.
 42. The method of claim 33, wherein the layerscomprise an inner layer, an outer layer, and a middle layer sandwichedbetween the inner and outer layers.
 43. The method of claim 42, whereinthe middle layer is magnetic.
 44. The method of claim 42, wherein theinner and outer layers are non-magnetic.